There have been conventionally known charging apparatuses for charging secondary cells multistage-connected in series.
For charging apparatuses of this type, a so-called multistage constant current charging method has been proposed which reduces influence of internal resistances of secondary cells and enables charging up to a state closer to full charge by charging the secondary cells with a specified constant current until voltages thereof reach a specified voltage and, thereafter, charging them with a current having a smaller value (for example, see Patent Document 1).
There is also known a charging apparatus including an overcharge protection circuit for preventing secondary cells from being overcharged. The overcharge protection circuit of this type includes discharge route circuits each including a discharge resistor and arranged in parallel in respective secondary cells and, if overcharge of any one of the secondary cells is detected, the secondary cell detected to be overcharged is connected to the discharge route circuit and forced to discharge and a charging current flowing into this secondary cell is bypassed to the discharge route circuit (for example, see, Patent Document 2).
Incorporation of this overcharge protection circuit into the charging apparatus brings about the following advantages in addition to protection against overcharge. Namely, in the case of charging secondary cells, which are difficult to produce while satisfying characteristics such as internal resistance values during charging periods, such as lithium polymer batteries connected in series at multiple stages, the overcharge protection circuit functions as a balancing circuit for balancing cell voltages among the respective secondary cells to align them with a fixed upper limit value. Thus, it is possible to charge the secondary cells while compensating for characteristic variations among the secondary cells. Also in the case of charging such secondary cells that cell voltages (internal resistances) during charging periods largely vary due to aged deterioration, the upper limit value of the cell voltages is kept constant regardless of the internal resistances of the secondary cells during a charging period at the start of use and during a charging period after a certain degree of aged deterioration. Thus, charging compensating for aged deterioration is possible.    Patent Document 1: JP-A-2003-87991    Patent Document 2: JP-A-H10-50352